Improved cortical-layer specificity of vascular space occupancy fMRI with slab inversion relative to spin-echo BOLD at 9.4 T

Cerebral blood volume (CBV)-weighted endogenous functional contrast can be obtained by the vascular space occupancy (VASO) technique. VASO relies on nonselective inversion for nulling blood signals, but the implementation of VASO at magnetic fields higher than 3 T is difficult due to converging T₁ values of tissue and blood water and a stronger counteracting blood oxygen level-dependent (BOLD) effect. To improve functional CBV sensitivity, we proposed to use VASO with slab-selective inversion (SI-VASO). Computer simulations showed that the SI-VASO approach significantly increases functional sensitivity compared to the original VASO in a stronger magnetic field with a shorter repetition time. To examine layer-dependent specificity, SI-VASO and spin-echo BOLD (SE-BOLD) functional magnetic resonance imaging (fMRI) experiments were performed on isoflurane-anesthetized cats during visual stimulation at 9.4 T. Unlike simultaneously acquired SE-BOLD signal, the SI-VASO signal peaked at 0.9 mm from the surface of the cortex and was localized to the middle cortical layer. The full-width at half maximal response across the cortex was narrower for SI-VASO than for SE-BOLD (1.7 mm vs. 2.5 mm, respectively), suggesting that SI-VASO is better localized to neuronally active sites than SE-BOLD fMRI. The magnitude of the SI-VASO change in the middle cortex was - 1.45% with our experimental parameters, corresponding to a relative CBV change of ∼ 8% when baseline CBV was assumed to be 5% in a two-compartment model. The signal response profile across the cortex, the calculated CBV change, and the time course of SI-VASO fMRI were similar to those of previously obtained CBV-weighted fMRI with contrast agent in the same animal model, suggesting that SI-VASO measures predominately functional CBV responses.